This invention relates to the preparation of substituted aromatic amines. More specifically, it pertains to a process for the electrolytic reduction of substituted nitro aromatic compounds to produce their corresponding amines.
Of the substituted aromatic amines, aminohydroxybenzoic acids are known to be useful as monomers in the preparation of polybenzoxazoles. Polybenzoxazoles can be prepared by the condensation of certain multifunctional aromatic compounds such as the aminohydroxybenzoic acids of the present invention. Polybenzoxazole fibers have high tensile and compressive strengths and thermal stability and are desirable for military, aerospace and other applications requiring rigid materials.
The reduction of nitro aromatic compounds to their corresponding amines is well-known. For example, U.S. Pat. No. 3,475,299 describes an electrolytic reduction of a nitro aromatic compound in an acidic medium in the presence of hydrogen sulfide. U.S. Pat. No. 3,424,659 discloses a process for electrolytically reducing nitro aromatic compounds in an electrolytic cell with an acidic catholyte and a basic anolyte. U.S. Pat. No. 3,475,300 describes a process for reducing nitro aromatic compounds in the presence of sulfuric acid.
All of the above processes relate to an electrolytic reduction in acidic medium. The acidic environment of the aforementioned processes may induce a Bamberger type rearrangement of reaction intermediates, especially at elevated temperatures. The acidic medium makes aromatic compounds susceptible to nucleophilic attack by moieties present in the solution such as water. Therefore, the presence of an acidic medium may lead to the formation of undesirable by-products if direct reduction of the nitro aromatic compound to its corresponding amine is desired. Thus, the selectivity of the electrolytic reduction is decreased.
The limited electrolytic reduction of nitro aromatic compounds in the presence of base has been previously described. For example, Brown and Warner, J.Phys.Chem., 27, 455-465 (1923) describe the reduction of o-nitrophenol by electrolysis to o-amidophenol. Probably o-aminophenol was intended in an alkaline medium in the presence of various metals such as zinc, lead and copper as the cathodic material. Belot et al., Tetrahedron Letters, Vol. 25, No. 47, 5347-5350 (1984) disclose the electro catalytic hydrogenation of nitro compounds to amines in an alkaline medium in the presence of Devarda copper and Raney nickel electrodes. Belot et al. teach that the reduction is very inefficient and produces unwanted azobenzene when a conventional copper electrode is employed. Organic Electrochemistry, M. M. Baizer & H. Lund, 2nd Ed., Marcel Dekker, Inc., 295-313 (1983) teaches that the electrolytic reduction of various nitro aromatic compounds in an alkaline medium most often yields dimers and other coupled products.
An electrolytic process is needed that would provide for the selective reduction of functionalized nitro aromatic compounds in basic media to their corresponding amines. A process is also needed that would provide a high current efficiency and thereby minimize the amount of power consumed by the reaction.